Sixpence was a unit of currency in the United Kingdom.
The Twelfth Doctor, accompanied by Bill Potts, payed sixpence to a waterman to enter the 1814 frost fair. (TV: Thin Ice)
The Tenth Doctor claimed to Queen Victoria that Rose Tyler was a feral child who he had purchased for sixpence in old London Town. (TV: Tooth and Claw)
During the London Event, Gwynfor Evans asked Jamie McCrimmon if he had a sixpence to purchase a bar of Camfield's Fairy Milk Chocolate from a vending machine they had found. Though Jamie responded in the negative, Evans tried the machine without inserting any money and discovered, to his surprise, there was a chocolate bar in the machine's open compartment. (TV: The Web of Fear)
Behind the scenes
As promoted in the Walls' Sky Ray lollies advertisement, sixpence (6d) was the price of a Sky Ray lolly.